I'm currently playing Nightfang as a player. Our DM gave it DR 5/cold iron for a fey/moon connection. We found that out from lore rolls and from actually fightining it. I don't know how much he changed it from the original (he said he made it 3.5 and upgraded the adventure for our 3 13th level characters) but it was a really tough monster, too tough to be called a mooncalf. As we were fighting it I taunted him by saying I would rename him for the sagas so that not even its true name would survive the fight. I just need to think of a good replacement name for a druidic giant bat squid.